If you have access to a map, I would scan it, then using a vector app like Inkscape (Xara in my case) recreate the map with your own stylistic elements (so it doesn't look like an exact copy of what you started from). Once this is done divide each specific post code sector into a variated color scheme to differentiate each sector - then apply your text labeling. Pretty straight forward I would think.

Oh, and welcome to the Cartographers' Guild!

GP